Lithographic apparatus, device manufacturing method and substrate holder

ABSTRACT

A substrate holder has burls having a height not less than 100 μm and at least 10 vacuum ports arranged within a central region extending to a radius of two thirds the radius of the substrate. Thereby concave wafers can be reliably clamped by generating an initial vacuum in a central region which exerts a clamping force tending to flatten the wafer and allowing the initial vacuum to deepen until the wafer is fully clamped.

[0001] The present application claims priority to European ApplicationNo. 02258833.9, filed on Dec. 20, 2002, the entirety of which is herebyincorporated into the present application by reference thereto.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a lithographic apparatus, amethod of manufacturing a device, and a substrate holder.

[0004] 2. Brief Description of Related Art

[0005] A lithographic apparatus is a machine that applies a desiredpattern onto a target portion of a substrate. Lithographic apparatus canbe used, for example, in the manufacture of integrated circuits (ICs).In that circumstance, a patterning device, such as a mask, may be usedto generate a circuit pattern corresponding to an individual layer ofthe IC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. In general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

[0006] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-67-067250-4, incorporated herein by reference thereto.

[0007] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in patents U.S. Pat. No. 5,969,441and WO 98/40791, which are incorporated herein by reference thereto.

[0008] To hold the substrate to the substrate table, a so-called burlplate may be used. A burl plate described in patent EP-A-0 947 884(which document is incorporated herein by reference thereto) comprises aplate with a matrix arrangement of protrusions, or burls, on one faceand a wall surrounding the matrix of burls. The burls all have a heightof 150 μm. Holes in the burl plate lead to a vacuum system whereby thespace below the wafer can be evacuated. The pressure differentialbetween the normal atmospheric pressure above the substrate and theevacuated region below clamps the substrate firmly to the burl plate.The vacuum ports are relatively numerous, e.g. 20 or more, and aredisposed in two concentric rings.

[0009] Other known designs of substrate holder have a relatively smallnumber of vacuum ports, e.g. 3 or 4. For example, U.S. Pat. No.5,923,408 discloses a substrate holder with three vacuum ports andprotrusions that have total height of not less than 550 μm—made up of anarrow section of diameter 100 μm and height 50 μm on top of a widersection of diameter not less than 1 mm and a height not less than 500μm. U.S. Pat. No. 5,324,012 discloses a pin chuck-type holder with asingle vacuum port. The pin-type protrusions are said to have a heightof from 10 μm to 500 μm but no specific examples are given. EP 1 077 393A2 describes substrate holders that have one, four or eight vacuum portsand various arrangements of pin-like protrusions, but does not disclosethe height of the pins. EP 0 803 904 A2 discloses a substrate holderthat has pins of a height between 17.8-30.5 μm and four vacuum ports ina central region. GB 2 149 697 A describes a vacuum chuck with aplurality of pin-type protrusions of 50 μm in height and six vacuumports.

[0010] The known designs of substrate holder suffer from the problemthat if a concave (dished) substrate is placed on them it fails to beclamped because the wide gap between the raised edges of the substrateand the surrounding wall means that no vacuum develops underneath thesubstrate. Substrates can become concave due to processes carried out onthem to build devices and may be discarded if they become too dished tobe clamped onto the substrate table. The need to discard such substratesreduces yield and throughput.

SUMMARY

[0011] One aspect of the present invention is to provide a substrateholder that can more reliably clamp concave substrates.

[0012] According to an aspect of the invention, there is provided alithographic apparatus comprising: an illumination system constructed toprovide a beam of radiation; a support structure to support a patterningdevice, the patterning device constructed to impart a cross-section ofthe beam of radiation with a pattern to form a patterned beam ofradiation; a substrate having a radius; a substrate holder for holdingthe substrate, the substrate holder including a plurality of protrusionsupstanding from a surface and having substantially coplanar extremities,a wall surrounding the plurality of protrusions, and a plurality ofvacuum ports opening into a space bounded by the wall; and a projectionsystem constructed to project the patterned beam of radiation onto atarget portion of the substrate, each of the plurality of protrusionshaving a height of not more than 100 μm, and the plurality of vacuumports being ten or more vacuum ports, each of the plurality of vacuumports opening into a central region of the space bounded by the wall,the central region having a radius of not more than 70% of the radius ofthe substrate.

[0013] By reducing the height of the protrusions (which are alsosometimes referred to as pimples or burls) and ensuring that arelatively large number of vacuum ports opens into a central region ofthe substrate holder, it is possible to ensure that a vacuum developsunder the substrate even when the substrate is significantly concave.The pressure differential across the substrate tends to flatten thesubstrate enabling the initial vacuum to deepen, increasing the pressuredifferential and further flattening the substrate. It is therefore onlynecessary to develop an initial vacuum under the central region of thesubstrate to successfully clamp a substrate. The initial vacuum issufficient to have a flattening effect on the substrate but need not beas deep as the vacuum developed when the substrate is clamped. Thenecessary depth of the initial vacuum will depend on the mechanicalproperties and curvature of the substrates to be clamped. Once thesubstrate has been clamped it is flattened against the tops of theprotrusions and the clamping effect is the same as if the substrate hadbeen flat in the first place.

[0014] It is preferred that the protrusions have a height of no lessthan 60 μm to ensure that the vacuum pressure under the substraterapidly becomes uniform when the substrate is clamped. It is mostpreferred that the height of the protrusions is in the range of from 70to 80 μm. With protrusions of such a height, the inventors havediscovered that silicon substrates of standard dimensions with acurvature of up to 800 μm across a 300 mm wafer can be successfullyclamped.

[0015] Preferably, the number of vacuum ports is in the range of from 20to 40, all of which open into the space within the central region. In aparticularly preferred embodiment of the invention, all of the vacuumports open into an annular region having an outer radius not more than70% of the radius of the substrate (about 100 mm for a 300 mm substrate)and an inner radius of not less than 40% of the radius of the substrate(about 60 mm for a 300 mm substrate).

[0016] According to a further aspect of the invention, there is provideda method of manufacturing a device, comprising: providing a substrate;providing a substrate holder having a plurality of protrusionsupstanding from a surface and having substantially coplanar extremities,a wall surrounding the plurality of protrusions and a plurality ofvacuum ports opening into a space bounded by the wall, each of theplurality of protrusions having a height of not more than 100 μm and theplurality of vacuum ports being ten or more vacuum ports, each of thevacuum ports being open into a central region of the space bounded bythe wall, the central region having a radius of not more than 70% of theradius of the substrate; holding the substrate by evacuating the spacebetween the substrate and a substrate holder; providing a beam ofradiation using an illumination system; imparting a cross-section of thebeam of radiation with a pattern to form a patterned beam of radiation;and projecting the patterned beam of radiation onto a target portion ofthe substrate.

[0017] According to yet a further aspect of the present invention, thereis provided a substrate holder to hold a substrate in a lithographicapparatus, the substrate holder comprising: a plurality of protrusionsupstanding from a surface of the substrate holder and havingsubstantially coplanar extremities; a wall surrounding the plurality ofprotrusions; and a plurality of vacuum ports opening into a spacebounded by the wall, each of the plurality of protrusions having aheight of not more than 100 μm, and the plurality of vacuum ports beingten or more vacuum ports, each of the vacuum ports opening into acentral region of the space bounded by the wall, the central regionhaving a radius of not more than 70% of the radius of the substrate.

[0018] According to yet a further aspect of the present invention, thereis provided a lithographic apparatus comprising: a substrate having aradius; means for holding the substrate; and means for projecting apatterned beam of radiation onto a target portion of the substrate.

[0019] According to yet a further aspect of the present invention, thereis provided a substrate holder to hold a substrate in a lithographicapparatus, the substrate holder comprising: means protruding from asurface for supporting a substrate; means for surrounding the meansprotruding from a surface; and means for holding a substrate on themeans protruding from a surface.

[0020] Although specific reference may be made in this text to the useof lithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

[0021] The terms “radiation” and “beam” used herein encompass all typesof electromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

[0022] The term “patterning device” or “patterning structure” usedherein should be broadly interpreted as referring to as a device orstructure that can be used to impart a projection beam with a pattern inits cross-section such as to create a pattern in a target portion of thesubstrate. It should be noted that the pattern imparted to theprojection beam may not exactly correspond to the desired pattern in thetarget portion of the substrate. Generally, the pattern imparted to theprojection beam will correspond to a particular functional layer in adevice being created in the target portion, such as an integratedcircuit.

[0023] Patterning device may be transmissive or reflective. Examples ofpatterning device include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable and which may ensure that the patterning device is at adesired position, for example with respect to the projection system. Anyuse of the terms “reticle” or “mask” herein may be considered synonymouswith the more general term “patterning device”.

[0024] The term “projection system” used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive optical systems, reflective optical systems, andcatadioptric optical systems, as appropriate for example for theexposure radiation being used, or for other factors such as the use ofan immersion fluid or the use of a vacuum. Any use of the term “lens”herein may be considered as synonymous with the more general term“projection system”.

[0025] The illumination system may also encompass various types ofoptical components, including refractive, reflective, and catadioptricoptical components for directing, shaping, or controlling the projectionbeam of radiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

[0026] The lithographic apparatus may be of a type having two (dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

[0027] The lithographic apparatus may also be of a type wherein thesubstrate is immersed in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

[0029]FIG. 1 depicts a lithographic apparatus according to an embodimentof the invention;

[0030]FIG. 2 is a plan view of a substrate holder in the apparatus ofFIG. 1;

[0031]FIG. 3 is a partial sectional view of a substrate holder clampinga wafer W in accordance with an embodiment of the invention;

[0032]FIG. 4 is a graph showing the vacuum pressure underneath thesubstrate in FIG. 3;

[0033]FIG. 5 is a partial sectional view of a substrate holder in aninitial stage of clamping a concave wafer in accordance with anembodiment of the invention; and

[0034]FIG. 6 is a graph showing the vacuum pressure under the wafer inFIG. 5.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

[0035]FIG. 1 schematically depicts a lithographic apparatus according toan embodiment of the invention. The apparatus comprises: an illuminationsystem (illuminator) IL for providing a projection beam PB of radiation(e.g. UV radiation or DUV radiation), which in this particular case alsocomprises a radiation source LA; a first support structure (e.g. a masktable) MT for supporting patterning device (e.g. a mask) MA andconnected to first positioning structure PM for accurately positioningthe patterning device with respect to item PL; a substrate table (e.g. awafer table) WT for holding a substrate (e.g. a resist-coated wafer) Wand connected to second positioning structure PW for accuratelypositioning the substrate with respect to item PL; and a projectionsystem (e.g. a refractive projection lens) PL for imaging a patternimparted to the projection beam PB by patterning device MA onto a targetportion C (e.g. comprising one or more dies) of the substrate W.

[0036] As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

[0037] The illuminator IL receives a beam of radiation from a radiationsource SO. The source and the lithographic apparatus may be separateentities, for example when the source is an excimer laser. In suchcases, the source is not considered to form part of the lithographicapparatus and the radiation beam is passed from the source SO to theilluminator IL with the aid of a beam delivery system BD comprising forexample suitable directing mirrors and/or a beam expander. In othercases the source may be integral part of the apparatus, for example whenthe source is a mercury lamp. The source SO and the illuminator IL,together with the beam delivery system BD if provided, may be referredto as a radiation system.

[0038] The illuminator IL may comprise an adjustor AM that adjusts theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

[0039] The projection beam PB is incident on a patterning device,illustrated in the form of the mask MA, which is held on the mask tableMT. Having traversed the mask MA, the projection beam PB passes throughthe lens PL, which focuses the beam onto a target portion C of thesubstrate W. With the aid of the second positioning structure PW andposition sensor IF (e.g. an interferometric device), the substrate tableWT can be moved accurately, e.g. so as to position different targetportions C in the path of the beam PB. Similarly, the first positioningstructure PM and another position sensor (which is not explicitlydepicted in FIG. 1) can be used to accurately position the mask MA withrespect to the path of the beam PB, e.g. after mechanical retrieval froma mask library, or during a scan. In general, movement of the objecttables MT and WT will be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the positioning structure PM and PW. However, in the caseof a stepper (as opposed to a scanner) the mask table MT may beconnected to a short stroke actuator only, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2.

[0040] The depicted apparatus can be used in the following preferredmodes.

[0041] In step mode, the mask table MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C in one go (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

[0042] In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

[0043] In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device maybe updated after each movement of the substrate table WT or in betweensuccessive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmablepatterning device, such as a programmable mirror array of a type asreferred to above.

[0044] Combinations and/or variations on the above described modes ofuse or entirely different modes of use may also be employed.

[0045]FIG. 2 is a plan view of a substrate holder 10 which is positionedon the substrate table WT to hold a substrate thereon during exposures.The substrate holder 10 comprises a flat circular plate, the upper faceof which is provided with an array of burls 12 and is bounded by a wall11, as seen in FIG. 3. The burls 12 support the substrate W and have atotal area of usually less than about 4% of the area of the substrate.Whilst for illustrative purposes the burls 12 are shown as arrayed in aregular rectagonal matrix, other arrangements are possible, e.g.concentric rings.

[0046] The burl plate is also provided with through-holes 13, in thisexample there are 24 through-holes, arranged regularly around twoconcentric rings 14, 15. Through-holes 13 line up with vacuum ports onthe substrate table WT and form vacuum ports for evacuation of the spacebelow the substrate W and bounded by the wall 11.

[0047] The substrate W is removed from the substrate holder 10 byturning off the vacuum and lifting it from below by pins which extendthrough further holes (not shown) in the substrate holder 10. Thesefurther holes may be surrounded by walls that rise to meet the substrateso that there is no leakage of air into the space under the substratevia these holes.

[0048]FIG. 4 is a graph of vacuum pressure |Pvac|, that is the magnitudeof the difference between the pressure in a space below the wafer W andnormal atmospheric pressure above. When a wafer W is clamped correctlyon the substrate table WT, the pressure underneath the wafer W in thearea within the wall 11 is at a uniform high vacuum level P1.

[0049]FIGS. 5 and 6 illustrate what happens when a concave wafer W′ ispresented to the substrate holder 10. At its outer edge, the curvatureof the wafer W′ means that there is a large gap between the wafer W′ andthe substrate holder 10 so that the pressure in this area is the same asabove the wafer and there is no clamping effect. However, becauseaccording to an embodiment of the invention the heights of theprojections 12 are reduced and the vacuum ports are arranged in acentral region of the substrate holder 10, a vacuum does develop in thecentral region below the wafer W′, as indicated by the solid curved linein FIG. 6. There is therefore a pressure differential across the waferW′ causing a clamping force, albeit initially small, that clamps thewafer W′ to the substrate holder 10 and also tends to flatten thesubstrate W′. The flattening of the substrate W′ reduces the gap betweenit and the substrate holder 10 allowing the vacuum in the central regionto deepen. This in turn increases the flattening force on the substrateW′ and consequently the substrate W′ is rapidly flattened and fullyclamped to the substrate holder 10. The vacuum level underneath thesubstrate W′ then reaches the normal level, as indicated by the dashedline in FIG. 6.

[0050] The inventors have determined that certain conditions on theheight of the projections 12 and the number and positioning of thevacuum ports 13 are satisfied in order to enable concave substrates tobe clamped. The height of the projections 12 should be sufficientlysmall so that there is some resistance to airflow inwards under a curvedsubstrate W′ placed on the substrate holder 10 to allow an initialvacuum to develop underneath the central portion. At the same timehowever, the projections 12 should not be so short that the area ofinitial vacuum is confined too close to the vacuum ports 13 and auniform vacuum level underneath the wafer cannot be obtained. Theinventors have determined that to ensure clamping of curved wafers W′the projections 12 should have a height no more than 100 μm. The heightbeing measured from the surface representing most of the area of thesubstrate holder. Thereby the space below a substrate resting on theprotrusions has a maximum depth of 100 μm (excepting where the vacuumports open). It may also be advantageous that the height is no less than60 μm to ensure the vacuum pressure under the substrate quickly becomesuniform when the wafer is fully clamped. Clamping is particularlyeffective if the burl height is in the range of from 70 to 801 μm. Asubstrate holder 10 with projections having a height of 75 μm was foundto reliably clamp substrates where the maximum curvature was up to 800μm for a 300 mm wafer.

[0051] For the number and arrangement of vacuum ports 13, it isnecessary that they be sufficient in number and distributed sufficientlyclose to the center of the wafer to generate an initial vacuum. However,the vacuum ports should not be too far from the edge of the burl plateto ensure that the clamping process of concave wafers W′ is initiated.The inventors have determined that there should be at least 10 vacuumports within a central region. The central region is preferably boundedby a circle of radius less than or equal to 70% of the radius d1 of thesubstrate, e.g. 100 mm for a 300 mm (diameter) substrate. There shouldbe no vacuum ports opening outside this central region. It isparticularly preferred that the vacuum ports open into an annular regionhaving an outer radius no more than 70% of the radius of the substrateand an inner radius no less than 40% of the radius of the substrate. Inthe described embodiment the vacuum ports are provided on rings 14, 15having radii d2, d3 of 90 mm and 70 mm respectively.

[0052] While specific embodiments of the invention have been describedabove, it will be appreciated that the aspects of the invention may bepracticed otherwise than as described. The description is not intendedto limit the aspects of the invention.

What is claimed is:
 1. A lithographic apparatus comprising: anillumination system constructed to provide a beam of radiation; asupport structure to support a patterning device, said patterning deviceconstructed to impart a cross-section of said beam of radiation with apattern to form a patterned beam of radiation; a substrate having aradius; a substrate holder for holding said substrate, said substrateholder including a plurality of protrusions upstanding from a surfaceand having substantially coplanar extremities, a wall surrounding saidplurality of protrusions, and a plurality of vacuum ports opening into aspace bounded by said wall; and a projection system constructed toproject said patterned beam of radiation onto a target portion of saidsubstrate, each of said plurality of protrusions having a height of notmore than 100 μm, and said plurality of vacuum ports being ten or morevacuum ports, each of said plurality of vacuum ports opening into acentral region of said space bounded by said wall, said central regionhaving a radius of not more than 70% of said radius of said substrate.2. An apparatus according to claim 1 wherein said height of saidprotrusions is not less than 60 μm.
 3. An apparatus according to claim 2wherein said height of said protrusions is in a range between 70 to 80μm.
 4. An apparatus according to claim 1 wherein said plurality ofvacuum ports includes a range of 20 to 40 vacuum ports, each of saidvacuum ports opening into said central region.
 5. An apparatus accordingto claim 1 wherein each of said plurality of vacuum ports open into anannular region having an outer radius of no more than 70% of said radiusof said substrate and an inner radius of no less than 40% of said radiusof said substrate.
 6. A method of manufacturing a device, comprising:providing a substrate; providing a substrate holder having a pluralityof protrusions upstanding from a surface and having substantiallycoplanar extremities, a wall surrounding the plurality of protrusionsand a plurality of vacuum ports opening into a space bounded by thewall, each of the plurality of protrusions having a height of not morethan 100 μm and the plurality of vacuum ports being ten or more vacuumports, each of the vacuum ports being open into a central region of thespace bounded by the wall, the central region having a radius of notmore than 70% of the radius of the substrate; holding the substrate byevacuating the space between the substrate and a substrate holder;providing a beam of radiation using an illumination system; imparting across-section of the beam of radiation with a pattern to form apatterned beam of radiation; and projecting the patterned beam ofradiation onto a target portion of the substrate.
 7. A substrate holderto hold a substrate in a lithographic apparatus, said substrate holdercomprising: a plurality of protrusions upstanding from a surface of saidsubstrate holder and having substantially coplanar extremities; a wallsurrounding said plurality of protrusions; and a plurality of vacuumports opening into a space bounded by said wall, each of said pluralityof protrusions having a height of not more than 100 μm, and saidplurality of vacuum ports being ten or more vacuum ports, each of saidvacuum ports opening into a central region of said space bounded by saidwall, said central region having a radius of not more than 70% of theradius of the substrate.
 8. A lithographic apparatus comprising: asubstrate having a radius; means for holding said substrate; and meansfor projecting a patterned beam of radiation onto a target portion ofsaid substrate.
 9. A substrate holder to hold a substrate in alithographic apparatus, said substrate holder comprising: meansprotruding from a surface for supporting a substrate; means forsurrounding said means protruding from a surface; and means for holdinga substrate on said means protruding from a surface.